IRAC Instrument Handbook - IRSA - California Institute of Technology

IRAC Instrument Handbook
part of their image is near the saturation value (typically either 45,000 DN in channels 1 and 2 or 60,000
DN in channels 3 and 4) then they should suspect surrounding pixels of being near saturation.
IRACWRAPCORR (wraparound correction)
This module uses the flag bits set by the previous module and attempts to correct the wraparound problem
(Figure 5.4). Note that both the sign truncation and the non-linearity wraparound, i.e., doughnuts, are
corrected in the pipeline. The sign truncation correction is made by
5.1.7 IRACNORM (Fowler sampling renormalization)
Acorrected = Auncorrec ted − 65535. (5.4)
IRAC data are taken with Fowler (multi) sampling in order to reduce read noise. This is done by nondestructively
reading the array multiple times (set by the Fowler number), and accumulating the sum into
an internal register. Since these reads are summed, the result must be divided by the number of reads in
order to get the actual number of DN. Additionally, when data are transmitted to the ground, a variable
number of least significant bits are discarded as a means of data compression (Figure 5.5). In order to
correct for the effects of bit-truncation and Fowler sampling the data are transformed by
A
out
ABARREL
Ain
× 2
= (5.5)
AFOWLNUM
where ABARREL is the barrel-shift keyword and AFOWLNUM is the Fowler number keyword. Note
that in normal usage the Fowler number and barrel shift actually used and commanded by the science
center are such that they cancel, i.e.,
ABARREL
2
= 1
AFOWLNUM
and hence observers should not be surprised if this module normally appears to do nothing.
Pipeline Processing 74 Level 1 (BCD) Pipeline
(5.6)